1. Field of the Invention
This invention relates to rf plasma sources and the use of such sources for cleaning surfaces in space, and more particularly to helicon wave plasma sources suitable for cleaning spacecraft thermal radiators and telescopes.
2. Description of the Related Art
There is a need for a low power, self-contained cleaning system for removing contaminants that build up on the exposed surfaces of a spacecraft, without damaging the device being cleaned. For example, thermal radiators are used to cool a spacecraft by radiating more energy than they absorb. This results from their having a high emissivity in the infrared (IR) wavelengths corresponding to blackbody radiation from the warm spacecraft, and a low absorbtance over the wavelengths of the solar spectrum.
Thermal radiators become contaminated in space due to the condensation of hydrocarbon vapors outgassed from organic materials carried onboard the spacecraft, such as adhesives, potting compounds, conformal coatings and thermal blankets. Ultraviolet light from the sun causes photopolymerization of the hydrocarbons, which would otherwise re-evaporate to some degree; this generates high molecular weight films that do not re-evaporate. These contaminants can greatly increase the radiator's solar absorbtance, and thus reduce its cooling capacity. To counter this, extra large radiator panels are typically used. The extra radiators not only add weight and cost to the spacecraft, but also cool the spacecraft excessively before they become contaminated. This requires valuable onboard electrical power to be used to heat the radiators; the greatest heating is needed during eclipse seasons, when power is least available. Near the end of the spacecraft life, the radiators exhibit a poor heat rejection performance that can cause the onboard electronics to be subjected to large thermal extremes, thereby reducing their lifetime and reliability.
Imaging optics such as telescopes that are used on spacecraft also become contaminated in this way. The condensed hydrocarbon vapors form a scum that causes absorption and scattering of the light being imaged by the telescope, blurring the images. This contamination process is significantly worsened if the telescope is designed to view the sun, which result in the photopolymerization mentioned above.
Telescopes that are cooled to cryogenic temperatures, on the order of tens of Kelvins, to permit observations in the infrared, are also subject to a buildup of surface contaminants. Such cryotelescopes suffer from condensation not only of hydrocarbon vapors, but also of water vapor, carbon dioxide, ammonia and other cryocondensible gases. The frozen gases absorb incident radiation, and in time become roughened by sublimation roughening, increasing their optical scatter. Cryotelescopes have previously been warmed to sublime the frozen gases. However, this renders the instrument "blind" during the sublimation process, and consumes a great deal of cryogen. The residual hydrocarbon contaminants have been simply allowed to accumulate.
Previous attempts to develop a cleaner for space borne telescope optics used high energy ion beams to remove the contaminants. However, this resulted in damage to the delicate optical surfaces because of ion beam sputtering. Radiators often use conductive coatings such as indiumoxide, and the potential for sputter damage to such coatings has also made ion beam cleaning inapplicable to radiators.
Another approach involves the use of an ultraviolet lamp to create ozone within the telescope tube. The ozone oxidizes hydrocarbon contaminants on the telescope optics. Unfortunately, this approach requires the telescope tube to be pressurized with oxygen gas, which imposes a substantial burden upon the spacecraft in terms of telescope mass, large oxygen tankage and cryogen loss due to convective warming during cleaning.
Another approach to cleaning spacecraft surfaces is described in U.S. Pat. No. 4,846,425 to Champetier and assigned Hughes Aircraft Company, the assignee of the present invention. This technique uses the negative charge that is typically accumulated on a spacecraft, which collects more active electrons than relatively inactive positive ions. Neutral oxygen is released from the spacecraft, ionized by the background space plasma, and drawn back to the spacecraft by its negative charging to react with the surface contaminants. A very large oxygen supply is required, however, because most of the oxygen escapes and is not drawn back to the spacecraft. This is because the oxygen must be ionized within a few debye lengths (about 10-100 m) from the spacecraft to be drawn back, and the majority of the oxygen is not ionized within this zone.
The use of plasmas is well known in ground applications for removing hydrocarbons. Nascent oxygen atoms and ions in the plasma oxidize the hydrogen and carbon atoms that make up the contaminants, and the reaction energy propels the volatile oxide from the contaminated surface. One type of plasma source that has been used for this purpose is based upon a Penning electron discharge; such a plasma source is described in U.S. Pat. No. 4,800,281, also assigned to Hughes Aircraft Company. Penning-type sources generally include either a filamentary cathode or a hollow cathode for achieving thermionic electron emission. However, filamentary cathodes predictably burn out over time, and are therefore unacceptable for use in spacecraft applications in which replacement of the cathode is not possible. For hollow cathodes, the electronic emissive material that is used to coat the hollow cathode is often incompatible with reactive gases such as oxygen that are desirable plasma fuels for cleaning optical surfaces.
Another plasma source that has been used for groundbased cleaning applications is the helicon wave source. This type of device operates by coupling externally generated electric and magnetic fields into a plasma that is confined by an axial magnetic field. An antenna consisting of two loops diametrically positioned on the outside of the source tube produces a transverse rf magnetic field, perpendicular to both the tube axis and a constant axial magnetic field. The rf field excites a helicon wave in the source tube, and energy is transferred from this wave to the plasma electrons The helicon wave theory is discussed in Chen, "Plasma Ionization By Helicon Waves", Plasma Physics and Controlled Fusion, Vol. 33, No. 4, 1991, pages 339-364. The use of helicon wave plasma sources for semiconductor cleaning is described in Singer, "Trends in Plasma Sources: The Search Continues", Semiconductor International, July 1992, pages 52-57.
Helicon plasma sources use versions of an rf antenna that have complicated wiring schemes to avoid establishing an rf magnetic field parallel to the source tube axis. This type of antenna, commonly referred to as a Nagoya Type III antenna, is described in Watari et al., "Radio Frequency Plugging of a High Density Plasm", Physics of Fluids, Vol. 21, No. 11, November 1978, pages 2076-2081. The over-all plasma sources are quite large and massive, and consume too much power and gas to be considered for spacecraft applications.
Other reactive plasma sources such as parallel-plate reactors are also used to clean hydrocarbons in ground applications. In addition to excessive weight and power consumption, those sources produce ion energies high enough to risk damaging optical surfaces.